Continuation of our studies in radioiodination has led to the finding that a two-phase process--substrate in neutral, aqueous buffer and I2 in chloroform or cyclohexane--may be one of the better ways of effecting radioiodine labelling. The reaction is presumed to occur at the liquid-liquid interface; this method appears not only to enhance the reactivity of I2 but to permit a level of control in degree and site of iodination heretofor unattainable for many cases. Studies have continued on the use of both soluble and insoluble radioiodine polymers for labelling of peptides and proteins. In some cases, significantly better survival of biological activity has been realized with these methods than with those in standard use.